Semiconductor laser diode assembly

ABSTRACT

There is disclosed a semiconductor laser diode assembly. A semiconductor laser diode emits a beam of light. A ring member has a heat sink formed integral therewith. The semiconductor laser diode is mounted on the heat sink through a sub-mount. A common electrode is formed integral with the ring member. The common electrode extends opposite to the heat sink. Lead electrodes are held by the ring member through an insulating material filled therebetween in an airtight manner. A stem unit formed by the ring member, the lead electrodes, and the insulating material to provide electrical connection to the semiconductor laser diode. A cap unit fixed on the stem unit for hermetically sealing the semiconductor laser diode. The cap unit has a window through which a beam of light emitted by the semiconductor laser diode is radiated. The semiconductor laser diode assembly is made compact in size and easy to manufacture, providing a source of laser beam adapted for CD players, CD-ROM players and so on.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor laser diode assemblies of thecan-seal type having a laser diode hermetically sealed in a spacedefined between a metal can and a stem assembled therewith, and moreparticularly to such assemblies which are reduced in size for use inoptical pickup devices such as CD players, LD players, and CD-ROMplayers.

Semiconductor laser diode assemblies conventionally used for opticalpickup devices for CD players etc. are generally constructed as shown inFIG. 7. As in the figure, the laser diode assembly of this kind has acap unit hermetically sealing a laser diode for protecting against theexternal ambient, moisture, etc. The laser diode assembly has a metalstem 21 having a heat sink 21a formed by, for example, cold forgingintegrally with the stem 21, and two through-holes 21b formed throughthe stem 21 as shown in FIG. 8. A laser diode 23 is mounted on the upperend of the heat sink 21a via a sub-mount 22. A common electrode 28 iswelded to the underside of the stem 21 to provide electrical connectionto the stem 21 and the heat-sink 21a. On the other hand, lead electrodes26, are held by the stem 21 via an insulating material such as a lowmelting glass 29 in a manner of being insulated therefrom. The laserdiode 23 has one electrode electrically connected via conductor wires,not shown, to the lead electrodes 26, and the other electrode formed inan underside thereof being electrically connected via the sub-mount 22and the stem 21 to the common electrode 28. The laser diode 23 ishermetically sealed in a space defined by the stem 21 and the cap 24. Toprovide hermetical seal, the cap 24 is conventionally welded onto thesurface of the stem 21.

The cap 24, on the other hand, has a window formed with a window glass25 at a top wall thereof, for transmission of light emitted by thesemiconductor laser diode 23. The window glass 25 is adhered to an innersurface of the cap 24 through a low melting glass as an adhesive, notshown, in an airtight manner. The cap 24 is attached onto the stem 21 bymeans of electric welding. In order to enhance weldability, a projection24b is formed in a bottom end of the flange 24a, i.e., the bottom of thecap 24 to provide linear contact with the surface of the stem 21, asshown in FIG. 8. The contact between the stem 21 and the cap 24 via theprojection 24b causes concentration of electric current passedtherethrough, with placing in the electrodes 31, 32 ensuring aconnection between the stem 21 and the cap 24.

For semiconductor laser diode assemblies as stated above, the stem 21 isusually formed integrally with the heat sink 21a through cold forgingfrom a thick sheet material, requiring a large scale of a fabricatingsystem. There is therefore a problem of incurring mounting-up offabrication cost, and accordingly inexpensive supply of such laser diodeassemblies is difficult.

Furthermore, there is a disadvantage that there are inevitablyencountered variations in contact between the welding projection 24b andthe surface of the stem 21, resulting in variations in concentration ofelectric current passed therethrough and hence nonuniformity of welding.Accordingly, it is difficult to provide uniformity of welding and hencesealability for semiconductor laser diodes.

To cope with this, it may be considered that the cap be press-fittedonto the stem to offer hermetic seal between the cap and the stem.However, such method has not yet been practiced, because if so donethere is high possibility of incurring cracks or breaks in the windowglass or the low melting adhesive glass, when a pressing force isapplied onto the top face of the cap.

For carrying out welding between the cap and the stem, a flange must beformed in the lower end of the cap, and an allowance margin has to begiven to the stem to cope with deviation in diametrical position of thecap relative to the stem. However, such structure hinders compactnessfor semiconductor laser diode assemblies, particularly in diametricsizes.

In particular, there is a recent demand of reducing the overall ordiametrical size for semiconductor laser diode assemblies, e.g., 3 mm orsmaller in applications such as for CD players, LD players, or CD-ROMplayers. However, in the conventional structure, there is a necessity ofproviding an allowance margin of at least 0.5 mm to a stem and a flangein a cap of not smaller than 0.4 mm. To this end, there is a limitationfor reducing the diametrical size of the package, to approximately 5.6mm or smaller, thus making difficult to miniaturize semiconductor laserdiode assemblies to a desired size.

Meanwhile, where a semiconductor laser diode assembly is employed in anoptical pickup device including a diffraction grating, a laser diodeassembly is entirely built into a holder 33, as shown in FIG. 9, byfixing the stem 21 onto the bottom of the holder 33. In such cases, adiffraction grating 32 is attached to an aperture formed in the holder33 by being pressed against the inner surface of the holder 33 by theforce of the spring 31. The spring 31 is held at the opposite end by atop face of the cap 24. However, the force of the spring 31 is usuallyset to approximately 3 to 5 kg for securing the difraction grating 32,which force is liable to cause cracks or breaks in a window glass or anadhesive glass fixing to the cap 24, hindering the sealability of thelaser diode assembly.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide asemiconductor laser diode assembly of a can-seal type, which is compactin size and easy to manufacture, providing a source of laser beamadapted for CD players, CD-ROM players and so on.

It is another object of the present invention to eliminate an allowancemargin of the stem unit for welding with the cap unit, making the laserdiode assembly compact.

It is a further object of the present invention to prevent occurrence ofcracks or breaks in a window glass or an adhesive glass for attachingthereof when the cap unit is fixed on the stem unit.

It is yet a further object of the present invention to provide a methodof manufacturing a semiconductor laser diode assembly which is compactin size and easy to manufacture.

In accordance with a first aspect of the present invention, there isprovided a semiconductor laser diode assembly comprising: asemiconductor laser diode for emitting a beam of light; a ring memberhaving a heat sink formed integral therewith, the semiconductor laserdiode being mounted on the heat sink through a sub-mount; a commonelectrode formed electrically connected with the ring member; a leadelectrode held by the ring member through an insulating material filledtherebetween in an airtight manner; a stem unit formed by the ringmember, the lead electrode, and the insulating material to provideelectricity the semiconductor laser diode; and a cap unit fixed on thestem unit for hermetically sealing the semiconductor laser diode, thecap unit has a window through which a beam of light emitted by thesemiconductor laser diode is radiated. With such structure, the stem canbe constituted by a ring member and lead electrodes secured to theinside of ring member via an insulating material such as a low meltingglass, wherein the ring member is easily formed by press working such asdrawing or blanking from a metal sheet. It is therefore possible toreduce the size of the stem small without necessitating complicatedworking such as forging with a thick plate.

Preferably, the ring member has a flange formed at a lower end thereofto provide constancy in height of the semiconductor laser diode withrespect to an upper surface of the flange, the flange having cut-outsfor positioning the ring member in place in a circumferential directionthereof. With such arrangement, where a semiconductor laser diodeassembly is built into a device such as a CD player, the flange of astem can be utilized as a reference plane such that a beam of light isconstantly and properly positioned by a simplified manner.

More preferably, the cap unit comprises a metal shell formed with thewindow therein, the metal shell has an inner peripheral surface thereoffitted over an outer peripheral surface of the ring member to providehermetic seal for the semiconductor laser diode. By doing so, it ispossible to omit a flange for welding from a cap unit, together with anallowance margin for welding from a stem unit. This also contributes toreduction of diametrical size of the laser diode assembly.

In accordance with a second aspect of the present invention, there isalso provided a semiconductor laser diode assembly comprising: a capunit fixed on the stem unit for hermetically sealing the semiconductorlaser diode, the cap unit has a window through which a beam of lightemitted by the semiconductor laser diode is radiated; and a reinforcingprojection annularly formed on a top face of the cap unit. With sucharrangement, there is almost no fear that cracks or breaks occur in awindow glass, i.e., the light-transmissible glass, when a pressing forceis applied onto the cap unit from the above, for example, duringpress-fitting the cap unit onto the stem unit.

In accordance with a third aspect of the present invention, there isfurther provided a cap unit for electronic component assemblycomprising: a metal shell cylindrically formed to have a top walltherein, the metal shell having an aperture in the top wall; alight-transmissible member adhered to an inside of the metal shell tocover the aperture for providing a window through which light is allowedto pass; and a reinforcing projection annularly formed on a top face ofthe metal shell. The cap unit may be utilized for other assembliesinvolving a device such as a phototransistor, a photodiode, and alight-emitting diode of metal cap types, besides semiconductor laserdiode assemblies.

In accordance with a fourth aspect of the present invention, there isprovided a method of manufacturing a semiconductor laser diode assemblycomprising the steps of: (a) preparing a stem unit, (a-1) forming a ringmember having a heat sink and a common electrode, together with forminglead electrodes, (a-2) mounting a semiconductor laser diode on the heatsink through a sub-mount, (a-3) provide electrical connection betweenthe semiconductor laser diode and a lead electrode, thereby preparing astem unit; (b) preparing a cap unit, (b-1) forming a metal shell havingan aperture in a top wall thereof, (b-2) attaching a light-transmissiblemember in an inner surface of the metal shell to cover the aperture forproviding a window through which a beam of light emitted by thesemiconductor laser diode is allowed to radiate, thereby preparing a capunit; and (c) fitting an inner peripheral surface of the metal shellover an outer peripheral surface of the ring member so that the cap unitis fixed on the stem unit in a manner providing hermetic seal for thesemiconductor laser diode. In such method, the cap unit is appropriatelypress-fitted onto the stem unit in a self-aligned manner. This eliminatethe necessity of an allowance margin as required in the conventionalwelding, providing compactness for the laser diode assembly.

The laser diode assembly can be easily and inexpensively manufactured byforming a metal sheet by press drawing and blanking into the ring memberin a manner such that the heat sink and the common electrode areintegrally formed therewith, and then filling an insulating material inthe ring member to airtightly hold the lead electrode and insulate thelead electrode therefrom.

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription and accompanying drawings which set forth illustrativeembodiments to which the principles of the invention are applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a sectional view of a semiconductor laser diode assemblyaccording to a first embodiment of the present invention, while FIG.1(b) is an enlarged view showing a laser diode and its surroundingportion;

FIG. 2 is a sectional view of a cap unit of the semiconductor laserdiode assembly of FIG. 1;

FIGS. 3(a) to 3(d) are sectional views for explaining the process ofassembling a cap unit onto a stem unit;

FIG. 4 is a partially enlarged sectional view of a cap unit of FIG. 1,which shows stress distributions presented by simulative testing.

FIG. 5 is a partially enlarged sectional view of a cap unit of acomparative example, which shows stress distributions presented by thesimulative testing;

FIG. 6 is a bottom plan view of a stem unit of a semiconductor laserdiode assembly according to a second embodiment of the presentinvention;

FIG. 7 is a sectional view of a conventional semiconductor laser diodeassembly;

FIG. 8 is a partially enlarged sectional view showing a weldingprojection of the conventional semiconductor laser diode assembly beforewelding; and

FIG. 9 is an explanatory view of a conventional semiconductor laserdiode assembly in a state that a diffraction grating is attachedthereto.

DETAILED DESCRIPTION

Referring to FIGS. 1(a) to 1(b), there is illustrated a first embodimentof a semiconductor laser diode assembly according to one embodiment ofthe present invention. The semiconductor laser diode assembly comprisesa stem unit 5 generally in a cylindrical form, and a cap unit 10 in areverse-bottomed tubular form which is fixed on the stem unit 5.

The stem unit 5 comprises a ring member 1 which is formed of an Fe-basedconductor metal. The ring member 1 is generally in a tubular form, whichhas a flange 1b extending outward from a lower end thereof, a heat sink1a extending centrally and upwardly from an upper end thereof, and acommon electrode 1c extending downward from an outer end of the flange1b. The ring member 1 defines therein an inner space in which aninsulating material, e.g., a low melting glass 2 is filled. Two leadelectrodes 3, 4 are held airtightly by the insulating material 2 in thering member 1 so that these lead electrodes 3, 4 respectively extendclose to the heat sink 1a. It is noted that the ring member 1 can beformed from an Fe-based metal sheet having a thickness of ranging from0.2 to 0.4 mm. Alternatively, the ring member 1 may be formed from aCu-based metal sheet instead of the Fe-based sheet.

A semiconductor laser diode 7 as a light source is mounted on a tipportion of the heat sink 1a via a silicon sub-mount 6. The laser diodeis so small, e.g., 200 μm by 150 μm, that it is previously bonded on thesub-mount 6 for facilitating assembling processes. The sub-mount 6mounted with the laser diode 7 is mounted on the tip portion of the heatsink 1a. In the present example, a photodiode for receiving light, notshown, is formed in the sub-mount as a silicon substrate so that theintensity of a laser beam emitted by the laser diode 7 is monitored bythe photodiode. The laser diode 7 and the photodiode have respective oneelectrodes provided in the backside thereof electrically connected viathe sub-mount to the common electrode 1c where electrical connection isto be externally made. On the other hand, the laser diode 7, togetherwith the sub-mount 6, has the other electrode electrically connected viaan Au wire 8 to the lead electrode 4, as shown in FIG. 1(b). Meanwhile,the photodiode has the other electrode connected via an Au wire 8 to thelead electrode 3.

Referring to FIG. 2, the cap unit 10 comprises a shell member 11 whichis generally in a reverse-bottomed tubular form. The shell member 11 isformed, e.g., from a Kovar sheet through press working such as drawingand blanking. The shell member 11 has an aperture 13 formed in a topwall thereof and a lower opening 14. The aperture 13 is covered by awindow glass (a light-transmissible member) 15. The window glass 15 isfixed to an inner surface encompassing the aperture 13. The fixation ofthe window glass 15 is done by the use of a low melting glass 16 as anadhesive, so that the window glass 15 is adhered to the shell member 11airtightly.

The cap unit 10 constructed as above is fixed on a stem unit 5 mountingwith the laser diode 7 such that a laser beam emitted from the laserdiode 7 is allowed to radiate through the window glass 15 toward theoutside of the laser diode assembly.

The cap unit 10 of the present invention has a reinforcing projection 12annularly formed on an outer top surface of the shell member 11 so thatit upwardly projects from the edge between the tubular side wall and thetop wall of the shell member 11. This structure is based on the findingfrom the results of simulative tests conducted by the present inventors.That is, it has been found that, where a cap unit 10 provided with areinforcing projection 12 is press-fitted to a stem unit 5, the cap unit10 is relieved of stresses to such an extent that no cracks or breaksoccur in a window glass 15 or a low temperature glass 16. This isbecause where stresses are induced by a downwardly pressing forceapplied onto the top wall of the cap unit 10, the stresses concentratein the reinforcing projection 12 of the shell member 11. This in turnresults in relief of stresses in the vicinity of the window glass 15 andthe adhesive glass 16, though tensile stresses are slightly caused inthe window glass 15. In this manner, the window glass 15 and theadhesive glass 16 are relieved of stresses by providing the reinforcingprojection 12 in the top face of the shell member 11.

The reinforcing projection 12 may be formed in a size of, e.g., a heightH of 0.03 mm and the width W of 0.25 mm, where the cap unit 10 has anoverall height of 3 mm and inner diameter of 2.2 mm with a wallthickness t of 0.15 mm. The reinforcing projection 12 may be formed tosuch a shape that the downwardly applied force on the cap unit 10 isreceived by the reinforcing projection 12 instead of the entire topsurface of the cap unit 10. The width W of the reinforcing projection 12is preferably determined to not greater than approximately twice thelateral wall thickness or the thickness t. This is because if the widthis excessively broad, the force will act almost on the entire topsurface, reducing the effect by the projecting portion.

For press-fitting a cap unit over the stem unit, an assembling apparatusis employed. The assembling apparatus is provided with a positioningtable 17 having a horizontal planar surface for placing thereon a stemunit 5, a cap guide 18 for guiding the cap unit 10, and pressing member19 having a planer surface which is vertically movable for pressing thecap unit 10 downward. With the assembling apparatus, a stem unit 5 isfirst placed, e.g., by a vacuum collet in position on the positioningtable 17, as shown in FIG. 3(a). Then, the cap guide 18 is lowered to aposition over the stem unit 5 that its guide face is axially alignedwith therewith, as shown in FIG. 3(b). Subsequently, a cap unit 10 isinserted into the cap guide 18 so that the cap unit 10 is rested at itslower opening on the shoulder of the stem unit 5, as shown in FIG. 3(c).In such a state, the pressing member 19 is moved downward to depress thetop wall, i.e., the reinforcing projection 12 of the cap unit 10, sothat the cap unit 10 is press-fitted in a self-aligned manner onto theouter peripheral surface of the stem unit 5, as shown in FIG. 3(d). Itis noted that the depth of fitting is controllable by determining thestroke of the pressing member 19. The outer diameter of the stem unit 5,i.e., the ring member 1, may be approximately 2.21 mm and the innerdiameter of the lower opening 10 of the shell member 11, beapproximately 2.20 mm. The pressing force of the pressing member 19 maybe approximately 17 kg.

Then, the results of the simulative tests are explained. The simulativetests were conducted to investigate on the distribution of stressesinduced by press-fitting of a cap unit onto a stem unit. The results ofthe test for the inventive structure and the conventional art as acomparative example are shown in respective partial sectional views ofFIG. 4 and FIG. 5. Incidentally, the test was made under conditions thatthe lower opening 14 of the cap unit 10 is completely held so as toavoid spreading of it outward. Each figure illustrates part of a shellmember 11, a window glass 15, and an adhesive glass 16. In the figures,the areas denoted by reference character A shows strong tensile stressesof 40 to 2.5 kg·f/mm², the areas by character B weak tensile stresses of2.5 to 0.952 kg·f/mm², the areas by character C a transitional state ofbetween tensile stresses and compressive stresses of 0.952 to -0.599kg·f/mm², the areas by character D weak compressive stresses of -0.599to -1.37 kg·f/mm², and the areas by character E strong compressivestresses of -1.37 to -40 kg·f/mm².

As is understood from FIG. 4, with the cap unit 10 formed with thereinforcing projection 12 according to the present invention, there aredistributed strong stresses A which are observed limited only to anextremely narrow area of a lower end portion of the window glass 15. Inparticular, the window glass 15 has a weak tensile-stress area B in alower portion with almost no stresses C applied to an upper portion nearand involving the adhesive glass 16. On the contrary, in theconventional art without an reinforcing projection as shown in FIG. 5,the window glass 15 has a strong stress area A broadly present therein.Both the window glass 15 and the adhesive glass 16 have a weak stressarea B almost entire area thereof. Further, a strong compressive-stressarea E is present in an upper portion of the shell member 11, whichstresses act to strongly attract the window glass 15. It is thereforeunderstood that in the conventional art there is higher possibility thatthe cracks or breaks are apt to occur in the window glass 15.

Further, tests were conducted as to airtightness for semiconductor laserdiode assemblies according to the present invention and the comparativeconventional art. In the tests, criterion of an airtightness of 5.1Pa·cc/sec was used as to whether leakage of air takes place or not. Thatis, where there is no leakage of air, the assembly is determined asacceptable. The inventive semiconductor laser diode assemblies in numberof 40 were subjected to testing, and none of them were unacceptable. Onthe other hand, the conventional assemblies in number of 40 were tested,and there involved two of unacceptable assemblies.

In the present invention, the stem unit 5 is formed by the cylindricalring 1 from a leadframe so that the lead electrodes 3, 4 are fixedthrough the insulator material 2 in the ring member 1, making itpossible to reduce the outer diameter of the semiconductor laser diodeassembly to, e.g., 3 mm or smaller. Further, there is no necessity ofimplementing complicated processes such as forging, thereby reducingfabrication cost.

In this manner, hermetic seal is provided by press-fitting the innersurface of the cap unit 10 onto the outer peripheral surface of the stemunit 5 in an self-aligned manner. Accordingly, there is no necessity ofproviding the stem unit with a welding margin without giving a weldingprojection in a cap unit, thus simplifying the structure as well as theprocess. In particular, the flange of the stem unit according to thepresent invention can be reduced to as small as approximately 0.25 mm,despite the flange of the conventional art requires, e.g., 1 mm orgreater. To this end, the semiconductor laser diode assembly of thepresent invention is reduced of its diametrical size by approximately1.5 mm. Incidentally, in order to provide hermetic seal more complete,it is desired that the outer peripheral surface of the stem unit and/orthe inner surface of the cap unit is electroplated with, e.g., SnPb, In,or Ag.

The employment of the cap unit 10 formed with the projecting portioneliminates occurrence of cracks or breakage in the window portion tothereby improve the reliability, even where a diffraction grating isheld by a spring in the conventional welded structure.

The cap unit 10 may likewise be applied to light-emitting orlight-receiving assemblies involving a window with hermetic seal, suchas for phototransistors, photodiodes, and light emitting diodes of themetal-cap type, besides semiconductor laser diodes.

Then, a second embodiment of a semiconductor laser diode assembly isexplained below. The second embodiment is different from the firstembodiment in that a ring member 1 has cut-outs 1d formed therein asshown in FIG. 6. The ring member 1 is formed by press working such asdrawing and blanking of an Fe-based metal sheet with a thickness of 0.2to 0.4 mm, similarly to the first embodiment.

A plurality of cut-outs 1d are provided in the flange 1b of the ringmember 1 so that the ring member 1 can be positioned in place relativeto a circumferential direction thereof. The ring member 1 has an innerspace thereof filled with an insulating material 2 such as a low meltingglass so that lead electrodes 3, 4 are held by hermetically sealing theinsulating material 2. Thus, a stem unit 5 is constituted by the ringmember 1, the insulating material 2, and the lead electrodes 3, 4.Incidentally, the cutout portion 1d may be formed in other shapes, e.g.,a shape straightly cut away arcuate portions, instead of the shape asshown in FIG. 6.

In assembling, the stem unit 5 is placed in position with reference tothe cut-outs 1d, and then a sub-mount 6 previously bonded with a laserdiode 7 is transported and mounted onto the heat sink 1 in place. Onthis occasion, the positioning of the sub-mount 6 is performed withreference to an upper surface of the flange 1b. By doing so, it ispossible to build a laser diode assembly into a proper position of,e.g., a CD player assembly with utilizing the upper surface of theflange 1b as a reference surface. It is satisfactory that the flange 1bhas a radial length of approximately 0.3 mm, because almost no deviationoccur laterally in a cap unit relative to the stem unit, and hence noallowance is necessitated in the stem unit for avoiding such deviation.

The laser diode 7 thus fixed on the stem unit 5 is hermetically sealedby a pre-assembled cap unit as in the first embodiment. This hermeticsealing is performed by fitting an inner surface of the cap unit onto anouter peripheral surface of the ring member. Such sealing is possible byvarious ways such as providing an electroplating layers using a softmetal to respective fitting surfaces, applying an adhesive such as anepoxy resin between the facing surfaces, besides press-fitting of thesesurfaces.

As described above, the stem unit is constituted by a ring member madefrom a leadframe so that lead electrodes are secured to an inside of thering member via an insulating material. No forge working is required forformation of the stem unit, thus simplifying the manufacturing process.The structure of the invention makes possible reduction in diametricsize of the laser diode assembly, as compared with the conventionalassembly.

Furthermore, the reinforcing projection contributes to preventionagainst occurrence of cracks or breaks in the window glass or theadhesive glass of the cap unit, even where a pressing force is appliedonto the top face of the cap member. Due to the presence of thereinforcing projection, it is possible to improve the airtightness forthe laser diode by press-fitting the cap unit to the stem unit in asimplified manner, instead of unstable electric welding. It is to beunderstood that various changes, modifications, and adaptations of theabove described embodiments may be practiced without departing from thespirit and scope of the invention as set forth in the appended claims.

What is claimed is:
 1. A semiconductor laser diode assembly comprising:asemiconductor laser diode for emitting a beam of light; a cylindricallyshaped ring member having a heat sink formed integral therewith, saidsemiconductor laser diode being mounted on said heat sink through asub-mount; a common electrode formed integral with said ring member; alead electrode held by said ring member through an insulating materialfilled therebetween in an airtight manner; a stem unit formed by saidring member, said lead electrode, and said insulating material so as toprovide electricity to said semiconductor laser diode; and a cap unitfixed on said stem unit for hermetically sealing said semiconductorlaser diode, said cap unit has a window through which a beam of lightemitted by said semiconductor laser diode is radiated.
 2. Thesemiconductor laser diode assembly of claim 1, wherein said sub-mount isformed of silicon for providing electrical connection between saidsemiconductor laser diode and said common electrode.
 3. Thesemiconductor laser diode assembly of claim 2, wherein a photodiode isformed in said sub-mount to monitor the intensity of a beam of lightemitted by said semiconductor laser diode.
 4. The semiconductor laserdiode assembly of claim 1, wherein said ring member has a flange formedat a lower end thereof to provide constancy in height of saidsemiconductor laser diode with respect to an upper surface of saidflange, said flange having cut-outs for positioning said ring member inplace in a circumferential direction thereof.
 5. The semiconductor laserdiode assembly of claim 1, wherein said cap unit comprises a metal shellformed with said window therein, said metal shell has an innerperipheral surface thereof fitted over an outer peripheral surface ofsaid ring member to provide hermetic seal for said semiconductor laserdiode.
 6. The semiconductor laser diode assembly of claim 1, whereinsaid cap unit has a reinforcing projection formed annularly and integraltherewith on a top face of said cap unit.
 7. A semiconductor laser diodeassembly comprising:a semiconductor laser diode for emitting a beam oflight; a stem unit having a heat sink formed integral therewith formounting thereon said semiconductor laser diode through a sub-mount; acommon electrode and a lead electrode formed in said stem unit toprovide electrical connection to said semiconductor laser diode; a capunit fixed on said stem unit for hermetically sealing said semiconductorlaser diode, said cap unit has a window through which a beam of lightemitted by said semiconductor laser diode is radiated, wherein said capunit including a) a light transparent member adhered to an inner surfaceof said cap unit to cover said window and b) a reinforcing projectionannularly formed on a top outer surface of said cap unit to projectabove said light transparent member.